A key process in developmental neurobiology is the manner by which axons projecting from newly born neurons choose paths to reach and innervate target organs. This proposal addresses the role of endothelin signaling as a critical mechanism that controls the projection of sympathetic axons from the superior cervical ganglia (SCG). New observations described in this proposal lead to a model in which vascular smooth muscle-derived endothelins provide guidance cues for sympathetic axons from the SCG to take a specific vascular trajectory, along the external carotid artery. A subset of SCG neurons express the type A endothelin receptor (ETA), which is essential for SCG axons to choose between distinct trajectories and to innervate their appropriate target tissues. In this application, I propose experiments to address and extend a model in which endothelin signaling, emanating from smooth muscle of the external carotid arteries, is a required guidance cue for SCG axons. Specific Aim1: To define the genetic requirements for endothelin signaling components in SCG axon guidance. This aim addresses the mechanism of endothelin ligand production by smooth muscle specifically of the external carotid arteries. Specific Aim 2: To define the tissue-specific functions of endothelin signaling components in SCG axon guidance. This aim addresses the extent to which ETA is intrinsically required by SCG neurons to respond endothelins in order to project along the external carotid arteries. Specific Aim3: To define the molecular basis of defective SCG outgrowth in Hoxa3 mutant mice. In this aim, I will evaluate how expression of endothelin signaling components is perturbed in Hoxa3 mutant mice, which have a comparable SCG outgrowth deficiency as seen in ETA-deficient embryos. Public health relevance: This research addresses the molecular basis of nerve-blood vessel congruency and the establishment of proper neural circuitry during development, both of which are relevant to the treatment for congenital defects in neural innervations, and for regeneration of neural circuitry following neurological diesease or injury. The principles learned will also apply to organ transplantation studies to achieve reinnervation of amputated axons to transplanted organs.